Add left widget panel to Launcher home when unfolded

Test: manual
Bug: 175939730
Change-Id: I9831e7fa95084db12953ec7cb2eb725e7549e01d

src/com/android/launcher3/WorkspaceLayoutManager.java

@@ -32,6 +32,8 @@ public interface WorkspaceLayoutManager {
     int EXTRA_EMPTY_SCREEN_ID = -201;
     // The is the first screen. It is always present, even if its empty.
     int FIRST_SCREEN_ID = 0;
+    // This panel is shown on the first page if the panel count is greater than 1.
+    int LEFT_PANEL_ID = -777;
 
     /**
      * At bind time, we use the rank (screenId) to compute x and y for hotseat items.